This disclosure is related to the field of pumping fluid through a pipe or conduit inserted into a wellbore drilled through subsurface formations. More specifically, the disclosure relates to methods for determining axial position of different fluids both within the conduit and an within annular space outside the conduit, and controlling movement of the fluids to avoid wellbore mechanical problems.
Pumping fluids through a subsurface wellbore includes using a pump disposed at the Earth's surface, or proximate the water surface for marine wellbores. Discharge of one or more selected types of fluid from the pump may be directed through a conduit or pipe disposed in the wellbore. The conduit may extend to the bottom (axially most distant from the surface end) of the wellbore. The pumped fluid moves through the interior of the pipe and may return through an annular space (“annulus”) between the pipe and the interior wall of the wellbore.
During construction of a wellbore, it may be desirable in certain circumstances to move different types of fluid through the pipe and into the annulus. For example, a “sweep” or limited volume of high viscosity fluid may be moved through the annulus to assist in removing drill cuttings from the wellbore. Alternately, a “pill” or limited volume of fluid may be used for other purposes such as to stop circulation loss (i.e., loss of fluid from the annulus into exposed formations) or to free stuck drill string or other tubular element.
During the course of wellbore drilling, various additives may be mixed into the drilling fluid in order to address different specific requirements, e.g., a lubricant to reduce friction, to reduce stuck pipe tenancies and to increase drilling rate (ROP). Weighting materials may be added to increase the fluid density (“mud weight”). In cases when such materials are added to the pumped fluid, it is useful to know the placement within the wellbore at any time of the fluid having the additives in order to better manage dynamic drilling parameters.
During completion operations, a casing (a pipe extending from the well bottom to the surface) or liner (a pipe extending from the bottom of the well to a selected depth, usually proximate the bottom of a previously installed pipe or casing) may be cemented in place in the wellbore. Cementing operations including pumping several different types of fluid in succession, including cement. The cement is typically pumped so that it either fills the annulus completely or is pumped to a selected depth in the annulus, depending on the design of the wellbore.
Irrespective of the type of fluids being pumped, it is valuable for the drilling unit operator to have information concerning the axial position within the annulus of each of the pumped fluids, the flow rate and flow regime (laminar or turbulent) of each of the fluids at various locations, and the hydrodynamic pressure exerted by the fluids in the annulus. Knowing the hydrodynamic pressure may be important to prevent either fluid influx from any permeable formations exposed to the annulus if the hydrodynamic pressure falls below the fluid pressure in such formations, or fluid loss from the annulus if the hydrodynamic pressure exceeds the fracture pressure of any one or more formations.
The ability to optimize flow rate within a safe operating “envelope” (i.e., a set of limiting operating parameters) may enable the wellbore operator to avoid problems and to maximize performance during wellbore construction operations.